2014-04-25 19:42

题目：C++中虚函数的工作原理？

解法：虚函数表？细节呢？要是懂汇编我就能钻的再深点了。我试着写了点测大小、打印指针地址之类的代码，能起点管中窥豹的作用，从编译器的外部感受下虚函数表、虚函数指针的存在。

代码：

1 // 13.3 How does virtual function works in C++? 2 #include 
     
       3 #include 
      
        4 using namespace std; 5  6 class A { 7 }; 8  9 class B {10 public:11     void f() {cout << "class B" << endl;};12 };13 14 class C {15 public:16     C();17 };18 19 class D {20 public:21     virtual void f() {22         cout << "class D" << endl;23     };24 };25 26 class E: public D {27 public:28     void f() {29         cout << "class E" << endl;30     };31 };32 33 int main()34 {35     D *ptr = new E();36     D d;37     E e;38     unsigned ui;39     40     cout << sizeof(A) << endl;41     cout << sizeof(B) << endl;42     cout << sizeof(C) << endl;43     cout << sizeof(D) << endl;44     cout << sizeof(E) << endl;45     // The result is 1 1 1 4 4 on Visual Studio 2012.46     ptr->f();47     // class with no data member have to occupy 1 byte, so as to have an address.48     // class with virtual functions need a pointer to the virtual function table.49     printf("The address of d is %p.\n", &d);50     printf("The address of e is %p.\n", &e);51     printf("The address of d.f is %p.\n", (&D::f));52     printf("The address of e.f is %p.\n", (&E::f));53 54     memcpy(&ui, &d, 4);55     printf("d = %X\n", ui);56     memcpy(&ui, &e, 4);57     printf("e = %X\n", ui);58     memcpy(&ui, ptr, 4);59     printf("*ptr = %X\n", ui);60 61     return 0;62 }